A conventional sewing machine drive device is designed as follows:
The electromagnetic clutch is excited by depressing the foot pedal so that it is electromagnetically coupled to the flywheel which is coupled to the rotor of a sewing machine drive motor which is rotated at all times, and the sewing machine is driven through the electromagnetic clutch thus coupled.
Furthermore, by changing the amount of depression of the foot pedal, an electromagnetic clutch exciting current from the motor speed control circuit is controlled, so that the speed of the sewing machine is controlled to a desired value according to the amount of depression of the foot pedal.
Thus, the conventional sewing machine drive device is disadvantageous in the following points: As the drive motor is rotating at all times whether the sewing machine is driven or stopped, the drive device consumes electric power uneconomically. Since the flywheel has a large moment of inertia, it is difficult to change the direction of rotation of the motor at a desired time. If a friction type electromagnetic clutch is employed, the service life of the drive device is reduced because the clutch is subject to wear.
In order to overcome the above-described difficulties, a sewing machine drive device has been proposed in Japan, in which a print motor is employed as the sewing machine drive motor and is driven by direct current, and the electromagnetic clutch is eliminated, and in which the print motor is directly started or stopped or driven at various speeds according to the operation of the foot pedal. The drive device has an excellent control characteristic because of the employment of the DC motor; however, it is still disadvantage in the following points: The brushes are subject to wear, which reduces the service life, thus requiring inspection and maintenance. Since the motor is started and stopped whenever the foot pedal is in operation, the start current is large. Accordingly, a large capacity power source must be employed, and the drive control circuit is bulky, which results in a high manufacturing cost of the drive device.